The invention applies to the field of optical data transmission using an optical transmission device which generates corresponding optical pulses when electrical data pulses are applied to it. The optical pulses can be transported to a receiver, for example using at least one optical transmission medium (e.g. an optical fiber) connected to the transmission device. The invention is aimed at an optical transmission device having at least one laser transmitter, whose operating point is regulated on the basis of a prescribed value.
WO 93/13576 discloses a transmission device having a plurality of laser diodes which can be electrically driven individually for the purpose of optical data transmission. The individual laser diodes are driven by a total current which is composed of a current (alternating current IAC) which is modulated on the basis of the (binary) data to be transmitted, and a direct current (IDC). The direct current is used for setting the mean optical output power (also called target power) and defines the operating point of the respective laser diode. The transmission device has a control loop which regulates the output power of all the laser diodes together such that the target power of the individual laser diodes is kept as constant as possible, for example by compensating for temperature-related fluctuations in their optical characteristics.
EP 0 507 213 A1 discloses a transmission device having a laser transmitter which emits light as a function of binary data signals at the input. A monitor receiver converts part of the emitted light to a monitor current. A control circuit regulates the mean optical output power of the laser transmitter to a constant value by evaluating the monitor current on the basis of a reference current provided by a reference current source. The specific way in which the magnitude of the reference current is determined is not explained in more detail.
However, significant differences in the optical behavior of individual laser transmitters mean that, during production of optical transmission devices having laser transmitters, tuning is necessary so that the laser transmitters have approximately the same optical properties. In this regard, it is conceivable for an analog control circuit to be used to vary the operating point of the laser transmitters using resistor decades or variable resistors (potentiometers). Subsequently, the resistor dimensioned in this manner for an optimum operating point can be soldered into the respective control circuit. This is relatively time consuming; furthermore, the laser transmitter needs to be gauged again for checking purposes after tuning effected in this manner.
The object of the invention is to create a transmission device which permits its optical properties to be tuned rapidly, permanently and with little effort.
This object is achieved by an optical transmission device having at least one laser transmitter which emits light as a function of data signals to be transmitted, having a monitor receiver which receives part of the emitted light and converts it to a monitor current, having a control circuit which regulates the mean optical output power of the laser transmitter to a constant value on the basis of a DC prescribed value by evaluating the monitor current, the DC prescribed value being obtained by tuning, and having a read only memory which permanently stores the prescribed value in the course of the tuning and supplies the prescribed value to the control circuit.
An essential aspect of the invention is thus that the DC prescribed values determined in a tuning operationxe2x80x94which is preferably carried out immediately after production has finished, while still at the factoryxe2x80x94are written to a memory as nonvolatile values. This read only memory supplies the prescribed values to suitable circuits which, on the basis of these prescribed values, generate a direct current, which is to be prescribed in each case, and apply it to the input of the control circuit. Hence, there is advantageously no need to provide or introduce separate resistors for the purpose of providing prescribable direct currents. The tuning operation can thus be significantly shortened since the final test can take place together with the tuning.
Advantageously, the DC prescribed value can represent at least one DC variable relevant to the operating behavior of the transmission device (namely the threshold current or the modulation direct current), said DC variable being applied to the input of the control circuit which regulates the optical output power of the laser transmitter to a constant value by evaluating the monitor current. In this context, the threshold current (also called ITH) is the minimum current which needs to flow for a laser to reach the laser state and emit laser pulses. The modulation direct current is used to set the modulation swing for generating the (binary) light or laser signals. In accordance with preferred refinements based on the invention, the threshold current or the modulation direct current can be fed into the laser transmitter on the basis of a further prescribed value (for the threshold current or the modulation direct current) using a controller. The DC prescribed value determines the mean optical output power in the case of this refinement of the invention too; in this context, the threshold current or the modulation direct current is the regulated current, while the other current in each case is controlled. Advantageously, the respective further prescribed value is also permanently stored in a non-volatile memory in the course of the tuning.
As a preference, a temperature compensation current can also be generated. This is produced on the basis of a further prescribed value which is obtained during the tuning or can be known previously and is permanently written in the non-volatile memory. The temperature compensation current can be used to compensate for the influence of temperature variations sensed by a temperature sensor, for example, on the controlled threshold current or the modulation direct current.
In one refinement of the invention which is preferred in design terms and with regard to the miniaturization of transmission devices, the memory is a component part of an integrated circuit which also contains circuits or circuit parts for driving the laser transmitter and/or regulating the mean optical output power of the laser transmitter.
Illustrative embodiments of the invention are explained further below with the aid of a drawing, in which: